The invention relates to an all-wheel steering for motor vehicles having manual front actuated wheel steering (preferably servo-assisted) and motor-driveable rear wheel steering which is actuated by means of a control (regulating device) as a function of the steering angle of the front wheels and/or other parameters through a comparison of set values and actual values of the rear-wheel steering angle.
Vehicles having all-wheel steering are known in principle.
All-wheel steering should make possible a deflection of the rear wheels in an opposite direction to the front wheels in order to increase the turning ability of the vehicle. Also, there should be the possibility of steering the rear wheels in the same direction as the front wheels but with smaller steering angles, in order to improve--at least subjectively--the directional stability of the vehicle at higher speeds and, in particular, during sudden steering motions used during crash avoidance maneuvers. By virtue of the deflection in the same direction, a larger slip angle, such as is necessary during cornering, is also obtained at the rear wheels at the very beginning of cornering.
In this context, controlling the rear wheels as a function of speed is known where at very low travelling speeds, the rear wheels are controlled in the opposite direction to the front wheels, while at higher travelling speeds, control in the same direction is effected.
It is possible in principle to additionally steer the rear wheels as a function of other vehicle parameters in order, for example, to automatically mitigate crosswind effects or swerving movements of the vehicle.
In such highly developed all-wheel steering systems, a sophisticated computer-controlled control or regulating device is normally used for the control of the rear wheels. In the case of a malfunction of this sophisticated device, it must be possible to make a switch to a computer-independent emergency control.
In this connection, provision has already been made to return the rear wheels automatically to their neutral central position as soon as a malfunction of the computer is detected.
However, this is disadvantageous, since certain extremely critical driving situations can arise if returning of the rear wheels takes place during driving situations within the threshold region of the controllability of the vehicle, for example during rapid cornering. In addition, there is the fact that the driver may be dangerously surprised by a sudden return of the previously deflected rear wheels to their central position.
It is therefore the object of the invention to create an all-wheel steering system which, in a simple manner in terms of construction, guarantees a high degree of driving safety even in the case of a failure or a malfunction of the sophisticated computer-controlled control or regulating device.
This object is achieved according to the invention by having the front wheel steering positively connected to a first double-acting piston-cylinder unit, the piston of which in its central straight-ahead position of the front wheel steering, opens a bypass connecting the two chambers of the first piston-cylinder unit. Rear wheel steering is provided with a self-locking-free motor drive, prestressed by a spring arrangement into its straight-ahead position of rear wheel steering. This motor drive is positively coupled to a second double-acting piston-cylinder unit, the swept chambers of which have the same size ratio as the swept chambers of the first piston-cylinder unit. The two piston-cylinder units are hydraulically coupled to one another via two lines which--in the case where each of the units has swept chambers of unequal size--one is arranged to connect the larger swept chambers with one another and the other to connect the smaller swept chambers together. A third piston-cylinder unit is arranged between these lines with its swept chambers having the same size ratio as the swept chambers of the aforementioned piston cylinder units. The larger swept chamber of the third piston-cylinder unit is connectable via shut-off valve to the line connecting the larger swept chambers of the two other piston-cylinder units and the other, or smaller swept chamber of the third piston cylinder unit is connectable via a further shut-off valve to the line connecting the smaller swept chambers of the two other piston-cylinder units. When these shut-off valves are closed, front wheel steering and rear wheel steering are hydraulically positively connected to produce a deflection in the same direction of front and rear wheels.
The all-wheel steering according to the invention makes it possible when the shut-off valves are open and the control or regulating device is functioning, to control the rear wheels according to any desired steering laws by means of the motor drive.
As soon as an error in the control or regulating device is detected, the said shut-off valves drop into their closing position and a hydraulic positive coupling between front wheel steering and rear wheel steering is thereby obtained. It is particularly advantageous that the steering angle of the rear wheels, which has been set prior to this, remains unchanged upon the closure of the shut-off valves when the vehicle is travelling through a curve and the front wheels are correspondingly locked. Upon alteration of the steering angle of the front wheels, the rear wheels are steered concomitantly in the same direction. The rear wheels are set automatically to their straight-ahead position by the spring arrangement as soon as the front wheels are set to their straight-ahead position, since here, the positive coupling between front wheel steering and rear wheel steering is cancelled because the piston of the first piston-cylinder unit associated with the front wheel steering opens the bypass connecting the swept chambers of this unit.
A further particular advantage of the invention consists in the fact that leakproofness and hence the ability of the hydraulic system (formed by the piston-cylinder units and the associated lines) to function, can be constantly checked during the error-free operation of the control or regulating device. Namely, during this operating phase, in which the shut-off valves which connect the chambers of the third piston-cylinder unit to the lines between the other piston-cylinder units, are open. The piston of the third piston-cylinder unit executes stroke movements which are determined by the steering movements both of the front wheel and of the rear wheel steering, provided that the hydraulic system is operating in error-free fashion.
Thus, according to a preferred embodiment of the invention, if the piston of the third piston-cylinder unit is positively coupled to a stroke sensor connected to the input side of the control or regulating device, or of the computer of the latter, then it is possible to check by means of the computer, as to whether the piston stroke is commensurate with the respective front and rear steering angles, which are detectable by means of further stroke sensors at the piston of the other piston-cylinder units and are fed to the input side of the computer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.